Numerous circular scanning lens systems have been developed and are frequently utilized in connection with raster scanning of a field of view, wherein a relatively short, narrow portion of the field of view is instantaneously projected onto a focal plane. During each raster line scan the projected portion of the field of view is deflected in a substantially straight line across the focal plane. For certain types of raster scan mechanisms, as the field of view is scanned across a line, there is a tendency for the image to rotate as it is projected on the focal plane. In the prior art, circular scanning lens systems have been synchronized with the deflection to remove the tendency for the field to rotate.
The prior art circular scanning lens systems have generally included many optical elements which introduce substantial intensity losses in transmitting the optical image from the field of view to the focal plane. Hence, the intensity of the image projected onto the focal plane is substantially reduced, whereby the lens system must have a relatively low effective f/number, which has deleterious effects on resolution and depth of field.
In one prior art configuration, a folded optic system is utilized. The folded optic system, together with the requirement for a large number of elements and the spacing required between these elements, results in a package that is rather large. In addition, the large number of elements and folded optic configuration causes difficulties in fabricating and aligning the lens system, thereby increasing the costs. While other circular scanning lens systems do not require folded optics, they do not involve the use of very large and heavy scanning prisms that are positioned in front of a simple lens. Still other circular scanning devices employ mechanically scanned prisms that are positioned in front of an array of derotating prisms, thereby providing a relatively lossy transmission medium, large space and weight requirements, and relatively high costs.